The present invention relates to a process for removing or destroying acid gas constituents such as NO.sub.x and SO.sub.2 present in combustion gases.
Flue gases resulting from the combustion of carbonaceous fuels contains substantial quantities of pollutants including nitrogen oxide and sulfur dioxide. These pollutants can combine with other substances found in the atmosphere to produce serious environmental hazards such as acid rain and smog. It is therefore desirable to destroy or remove these pollutants before they are dispersed into the atmosphere.
There are two types of methods for removing or destroying acid producing constituents from combustion systems. One type known as combustion modification requires control over the combustion reaction producing pollutant. These techniques have generally achieved 50-60% reduction in NO.sub.x emissions from conventional combustion systems.
A specific type of combustion modification, known as re-burning, has achieved reductions approaching seventy percent. Using this technique, a secondary fuel source is introduced downstream of the primary combustion zone to achieve reductions of NO.sub.x. This technique, however is disadvantageous in that it requires large amounts of secondary fuel to accomplish the re-burning of NO.sub.x. This method is not effective when more than seventy percent removal of effluent NO.sub.x is required.
A second type of removal method is known as post-combustion clean-up. In this method, the pollutant is removed down-stream of its formation. These techniques are more complex and expensive but are useful when NO.sub.x reduction levels higher than 70% are necessary. In some systems a dry scrubbing sorbent or an aqueous sorbent such as an active metal chelate are typically used to remove NO.sub.x and other pollutants. One system proposed by Walker in U.S. Pat. No. 4,820,391 discloses scrubbing with an aqueous sorbent containing a metal chelate and regenerating the sorbent including the use of electrodialysis.
One other system, illustrated in U.S. Pat. No. 4,878,442 to Yeh et al., includes the combination of a low pollutant burner with a scrubber and regeneration system. This system, though obtaining high removal of NO.sub.x, involves the complicating aspects of the two combined systems.
In a study on the feasibility of ammonia-based post combustion NO.sub.4 control (EPRI CS-2713, November, 1982), a peak NO.sub.x removal efficiency of 55% was predicted because of high levels of ammonia carryover. Ammonia is a pollutant and its presence can result in corrosive ammonia salts. Although higher removal efficiencies have been attained under well-controlled laboratory conditions, temperature gradients, ammonia-flue gas mixing limitations, and poor flow distributions all increase ammonia carryover, which limits the removal efficiency.
In one other research effort (Behbahani et al., Combustion Science and Technology, (1983), Vol. 30, 289-302), nitrogen oxides are destroyed by nitrogen atoms generated in a plasma jet. This research effort investigates the removal of NO from an unheated inert gas stream by passing such flow within about two centimeters of the discharge of the plasma jet containing nitrogen. Nitrogen oxide removal of less than 50% at efficiencies below 30% are reported.